For someone who sings big songs in an unapologetically open-lunged manner, Manilow speaks quietly in interviews, often more mumbling than mellifluous.

And, although he’s known for sugar in his songs, he can be a little salty in person; he’s from New York, after all.

But perhaps the biggest surprise about Manilow, 70, is this: He has always wanted to make a first-class Broadway musical.

“I’ve been kind of imprisoned in the pop music world, very happily, but there are these rules that you need to adhere to in pop music,” Manilow said. “There is a certain brick wall that you hit. But this gave me the opportunity to go way, way beyond what I’ve been doing for 30 years.”

The result of that artistic stretch is Harmony, with a book and lyrics by his longtime collaborator, Bruce Sussman. It’s a show that has been gestating for two decades, including a 1997 run in California and a planned 2004 Broadway engagement, which was foiled when its lead producer announced that he was millions short of capitalization.

But Manilow never gave up on the show.

The Alliance Theatre in Atlanta will present Harmony in a monthlong run beginning Sept. 6. Tickets are selling briskly — more evidence of Manilow’s continued popularity.

The long wait between productions seems to have done nothing to dull the two creators’ ardor for Harmony, which tells the story of the Comedian Harmonists, a vaudevillian German sextet whose rise to international fame was interrupted by the Nazis’ rise to power.

Sussman, 64, said he came across the Harmonists in 1991, when he saw a documentary that explored their story and left him wanting more.

“This is a show about the quest for harmony in what turned out to be the most discordant chapter in human history,” Sussman said.

As for music, Sussman’s first call was to Manilow, with whom he had collaborated on the ridiculous, and ridiculously catchy, 1978 hit Copacabana and scores of other songs since the two met in the early 1970s.

Manilow recalled that, when Sussman first mentioned the Harmonists, it bothered him that he hadn’t heard of them.

“They were the architects of the kind of popular singing that we all grew up loving,” said Manilow, mentioning groups such as the Manhattan Transfer and Take 6. “They were huge. How did we miss these guys?”

In a recent rehearsal, Manilow stood at the piano, giving notes on an actor’s key and the phrasings of songs. He listened intently as the cast worked its way through How Can I Serve You, Madame? — a waltz that includes references to Hamlet, falsetto singing and complex harmonies. In short, Top 40 it isn’t.

Indeed, Manilow said, the songs in Harmony had to be “more authentic” than other things he’d written in his career, which began many years ago in Brooklyn with an eye always cast toward Broadway.

“This is not anything new for me,” he said. “This is what I’ve loved to do ever since I was a kid.”

Then he added: “I know how to do it. I just have never been asked to do it.”